1. Technical Field
The present disclosure relates a method for preparing caprolactam, and more particularly to a method for preparing caprolactam with a simplified process.
2. Description of Related Art
Caprolactam is an important raw material for the production of nylon 6 fibers and resins. In order to produce high quality caprolactam to satisfy the production of Nylon-6 or polymerization of the resin, there are many purification steps in the traditional production process. These purification steps comprise an organic solvent extraction, alkaline washing, back-extraction of caprolactam solution with water (water-back-extraction), steam stripping of removing organic solvent, ion exchange, and hydrogenation reactions, and finally the process is completed with evaporation and distillation steps to produce a high quality caprolactam. For example, to produce caprolactam in the industry, U.S. Pat. No. 7,700,767 discloses, under the oleum-catalyzed condition, cyclohexanone oxime was subjected to Beckmann rearrangement reaction, then the reaction mixture was neutralized with ammonia and subsequently was subjected to solvent extraction with benzene. The organic phase portion of benzene was collected, then undergone alkaline washing and water-back-extraction. Benzene was removed by steam stripping to obtain an aqueous solution of caprolactam with a concentration of about 30 to 40 wt %. The process was followed by ion exchange treatment system to remove inorganic salt impurities in caprolactam aqueous solution. Later, with the presence of hydrogen and a nickel-containing catalyst, unsaturated organic impurities in the caprolactam aqueous solution was reacted with hydrogen. Finally, the caprolactam solution was evaporated and distillated to obtain caprolactam products.
European Patent No. 411,455 discloses a continuous caprolactam purification method using cyclohexanone oxime subjected to Beckmann rearrangement, neutralization with ammonia, solvent extraction of benzene, steam stripping to remove the organic solvent, and other traditional manufacturing processes, and finally an aqueous solution form of caprolactam was subjected to the three-phase (vapor-liquid-solid) hydrogenation fixed bed operating system. In addition, U.S. Pat. No. 5,539,106 discloses a hydrogenation purification method for a caprolactam-water mixture, in which the caprolactam-water mixture is first contacted with the gaseous hydrogen, followed by contacting a hydrogenation catalyst for hydrogenation purification in a fixed bed.
It can be seen that, in the existing hydrogenation steps of the caprolactam manufacturing method, an aqueous solution of caprolactam is used for the reaction. Therefore, the steps of water-back-extraction are necessary to remove organic solvents in the aqueous solution. It consumes large amounts of steam energy. Moreover, ion exchange step which is carried out prior to the hydrogenation reaction in conventional manufacturing process requires frequent replacement and regeneration of ion exchange resins with increased production cost and the subsequent waste-water treatment problems.
Therefore, how to simplify the manufacturing process to produce high-quality caprolactam has become the issue to be resolved anxiously.